Mg or Mg-Alloy Housing and Method for Producing the Same

ABSTRACT

A surface of a Mg or Mg-alloy housing of the present invention is subjected to wet blast using a mixture containing a chemical conversion treatment agent and an abrasive, whereby a chemical conversion film is formed on the surface of the Mg or Mg-alloy housing. Because of this, a chemical conversion film is formed simply on the surface of a molded product made of Mg or a Mg-alloy to reduce the correction by puttying, whereby a Mg or Mg-alloy housing is provided in which a decrease in yield and an increase in cost can be prevented.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Mg or Mg-alloy housing in which a chemical conversion film is formed simply on a surface of a molded product made of Mg or a Mg-alloy so as to improve the yield of coating, and a method for producing the Mg or Mg-alloy housing.

2. Description of the Related Art

Recently there are increasing chances that mobile equipment such as a notebook computer and a personal digital assistant (PDA) is carried for outdoor use. Along with this, there is an increasing demand for the miniaturization and the reduction in weight and thickness of equipment, and hence, the reduction in thickness of a housing occupying 30% of the entire mass of the equipment is required. However, in the case where the equipment is reduced in thickness, it is difficult to maintain sufficient strength with a resin housing. Furthermore, the amount of heat generated by a microprocessor unit (MPU) of a notebook computer is increasing. The heat generation amount during AC driving is planned to be 13 to 16 W, so that the development of a housing having high cooling performance is urgently necessary.

As a solution to the above-mentioned problems, light metal such as Al, Mg, a Mg-alloy, or the like having low gravity and high heat conduction is becoming a focus of attention as a housing material. In particular, Mg or a Mg-alloy is light, i.e., has a gravity of about 70% of Al, and also is excellent in recyclability. As Mg or a Mg-alloy a Mg—Al—Zn system, a Mg—Al—Mn system, a Mg or Mg—Al—Si system, a Mg—Al—rare earth element (RE) system, and the like have already been put into practical use. These materials are molded by die casting, Thixo molding, sheet metal processing, or the like, and subjected to a chemical conversion treatment, and the chemical conversion product thus obtained is subjected to a coating treatment. Die casting refers to a method for injecting molten metal into a mold under pressure to obtain a molded product. Thixo molding refers to a method for extruding semi-molten or molten metal into a mold at about 250° C. from an extruder at about 600° C., thereby obtaining a molded product.

The chemical conversion treatment refers to a treatment of forming a chemical conversion coating film having a corrosion resistant function on a surface of Mg or a Mg-alloy According to this treatment, Mg or a Mg-alloy is soaked in a chromic acid solution mainly containing a hexavalent chromium solution, a non-chromic acid solution containing no hexavalent chromium, or the like, whereby a coating film of a chromium complex salt of chromic acid (in general, xCrO₃·yCr₂O₃·2H₂O, Cr(OH)₂·HCrO₄·2H₂O, etc.) is formed on the surface of the Mg or the Mg-alloy After the chemical conversion treatment, a coating treatment is performed. Methods for the chemical conversion treatment are proposed by JP11(1999)-131255A and JP2000-96255A.

There are the following problems in producing a product using Mg or a Mg-alloy as a material. A molded product obtained by die casting, Thixo molding, or the like has burr. At present, the burr is removed manually or mechanically by punching with a press, cutting, or the like, any of which is a cumbersome operation. There is also a method for removing the burr by shot blast of air-spraying grains onto a molded product. In this case, there are the following problems. Dust particles of Mg, a Mg-alloy or grains are likely to be generated and scatter, resulting in poor workability and high danger of explosion of dust particles.

Furthermore, a pretreatmnet is required for performing the above-mentioned chemical conversion treatment. The general chemical conversion treatment is performed as follows. The above-mentioned molded product is degreased, and then, washed with an acid. Then, the molded product is subjected to etching so as to remove a releasing agent used during molding. Then, a surface adjustment treatment is performed in which the surface of the molded product is polished to obtain satin finish so that a coating film of a chromium complex salt of chromic acid is likely to be formed on the surface of the molded product. The above-mentioned treatments correspond to a pretreatment. Then, the molded product is soaked in the above-mentioned chromic acid solution or non-chromic acid solution to perform a chemical conversion treatment. Then, the resultant molded product is washed with water, and dried at about 70° C., whereby a chemical conversion coating film is cured. Thus, the pretreatment of the chemical conversion treatment needs to have several processes such as degreasing, washing, a surface adjustment treatment by etching, and the like, which makes the operation cumbersome, complicates a treatment apparatus, and increases a treatment cost.

In the molded product produced by die casting, Thixo molding, or the like, molding defects such as a draw, a void, a hot water flow, and the like occur. During inspection of the molding defects (draw, void, flow, etc.), the molded product is partially buffed up manually so as to make it easy to see the molded product, and thereafter, a visual inspection is performed. At this time, buffing is performed manually which makes it cumbersome to perform the inspection, and may generate dust particles during buffing. As the method for adjusting a surface such as buffing, JP2003-284457A proposes a wet blast treatment. Furthermore, JP2000-263442A proposes a method for continuously performing the wet blast as a pretreatment of the chemical conversion treatment.

The above-mentioned conventional techniques use wet blast as a pretreatment of the chemical conversion treatment, and the number of processes and the like are similar to those of a treatment using a conventional chemical treatment, so that the effect of improvement in cost and the like is small. Furthermore, a defective molded product needs to be corrected by puttying before the chemical conversion treatment, decreasing a yield and increasing a cost.

SUMMARY OF THE INVENTION

Therefore, with the foregoing in mind, it is an object of the present invention to provide a Mg or Mg-alloy housing in which a chemical conversion film is formed simply on a surface of a molded product made of Mg or a Mg-alloy so as to reduce the correction by puttying, a coating yield is enhanced, and a cost can be reduced, and a method for producing the Mg or Mg-alloy housing.

A Mg or Mg-alloy housing of the present invention is characterized in that a surface of the Mg or Mg-alloy housing is subjected to wet blast using a solution containing a chemical conversion treatment agent and an abrasive, whereby a chemical conversion film is formed on the surface of the Mg or Mg-alloy housing.

A method for producing a Mg or Mg-alloy housing of the present invention is characterized in that a surface of a Mg or Mg-alloy is subjected to wet blast using a solution containing a chemical conversion treatment agent and an abrasive, whereby unevenness due to the blast and a chemical conversion film are simultaneously formed on the surface of the Mg or Mg-alloy housing.

These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a process flow in Example 1 of the present invention,

FIG. 2 is a diagram showing a process flow in Comparative Example 1.

FIG. 3 is a surface observed photograph of a notebook computer housing in Example 1 of the present invention.

FIG. 4 is a surface observed photograph of a notebook computer housing in Comparative Example 1.

FIG. 5A to 5C are surface observed photographs showing results of a cross-cut test in Example 1 of the present invention.

FIG. 6A to 6C are surface observed photographs showing results of a cross-cut test in Comparative Example 1.

FIG. 7A to 7C are surface observed photographs obtained by evaluating the corrosion resistance by a salt spray test (SST) in Example 1 of the present invention.

FIG. 8A to 8C are surface observed photographs obtained by evaluating the corrosion resistance by a salt spray test (SST) in Comparative Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, the surface of a Mg or Mg-alloy housing is subjected to wet blast using a mixed solution containing a chemical conversion treatment agent and an abrasive to form a chemical conversion film together with unevenness due to the blast on the surface of the Mg or Mg-alloy housing, which reduces the correction by puttying, thereby preventing a decrease in yield and an increase in cost. More specifically when the mixed solution containing the chemical conversion treatment agent and the abrasive is injected onto the surface of the Mg or Mg-alloy housing by wet blast, the surface is scraped off with the abrasive, and simultaneously, an appropriately thick and dense calcium phosphate-magnesium phosphate complex coating film is generated due to the reaction between Mg and the chemical conversion treatment agent. Thus, a film of an oxide or a hydroxide that degrades bare corrosion resistance, rust prevention, coating adhesion, and coating corrosion resistance is not formed on the surface of the Mg or Mg-alloy housing; instead, a calcium phosphate-magnesium phosphate complex coating film can be formed thereon in which calcium and phosphorus contribute to the enhancement of bare corrosion resistance of the chemical conversion film, and manganese contributes to the enhancement of coating adhesion. Furthermore, a draw, a void, a flow, and the like on the surface of a chemical conversion product are eliminated, which makes it unnecessary to perform correction by puttying before coating, and enables a cost to be reduced.

The inventors of the present invention have earnestly studied so as to solve the above-mentioned problems, and consequently found that, for example, as a rust prevention treatment and a pretreatment of a coating treatment of a housing made of Mg or a Mg alloy a chemical conversion treatment and wet blast are performed simultaneously with respect to the Mg or Mg alloy whereby the above-mentioned problems can be solved. Herein, wet blast refers to a treatment in which a liquid and an abrasive are mixed, and the mixture is injected under a pressure in a range of 0.1 to 0.3 MPa.

It is preferable that the chemical conversion treatment agent used in the present invention contains calcium ions, manganese ions, and phosphoric acid ions, and it is also preferable that wet blast is performed using a solution in which an aqueous liquid containing an oxidation accelerator and an abrasive such as alumina are mixed. In the wet blast, surface polishing and chemical conversion coating of a housing are performed simultaneously Therefore, a formed coating film is also polished; however, a sufficient coating film can be obtained by satisfying the above conditions.

It is preferable that at least one particulate material selected from alumina, zirconium, glass, and resin is used as the abrasive. Furthermore, as the resin, melamine resin, urea resin, polyester resin, phenol resin, epoxy resin, urethane resin, and the like can be used. It is preferable that an average particle diameter of the abrasive is in a range of 10 to 300 μm.

As a calcium ion source used for a chemical conversion treatment agent to be injected, one kind or at least two kinds of calcium nitrate, calcium nitrite, calcium thiosulfate, tetracalcium phosphate, and the like can be compounded. Furthermore, as the manganese ion source, one kind or at least two kinds of manganese carbonate, manganese nitrate, manganese hydrogen phosphate, manganese biphosphate, manganese fluoroborate, and the like can be compounded. Furthermore, as the phosphoric acid ion source, one kind or at least two kinds of orthophosphoric acid, condensed phosphoric acid, phosphorous acid, hypophosphorous acid, and the like can be compounded.

Furthermore, as the oxidation accelerator, sodium chlorate, sodium hypochlorite, and the like can be used. The oxidation accelerator can enhance the reactivity between the Mg or Mg-alloy and the above-mentioned respective component ions during the chemical conversion treatment, whereby a chemical conversion coating film with satisfactory bare abrasion resistance can be formed more conveniently Furthermore, the preferable compounded amount of the oxidation accelerator is in a range of 0.02 to 2 g/L so as to stably obtain a chemical conversion coating film with the above-mentioned performance.

According to the present invention, when a mixed solution of a chemical conversion treatment agent and an abrasive is injected onto the surface of Mg or a Mg-alloy by wet blast, the surface is scraped off with the abrasive, and an appropriately thick and dense calcium phosphate-manganese phosphate complex coating film is generated due to the reaction between the Mg or Mg-alloy and the chemical conversion treatment agent. Thus, a film of an oxide or a hydroxide is not formed on the Mg or Mg-alloy surface, and instead, a calcium phosphate-manganese phosphate complex coating film can be formed. It is preferable that the adhesion amount of the complex coating film formed by the chemical conversion treatment is 5 mg/m² to 50 mg/m² in terms of a Ca weight, 3 mg/m² to 25 mg/m² in terms of a Mn weight, and 30 mg/m² to 100 mg/m² in terms of a phosphorus weight.

Furthermore, it is preferable that the proportion of the chemical conversion treatment agent is 70 to 90% by mass, and the proportion of the abrasive is 10 to 30% by mass.

A chemical conversion coating film is formed on the Mg or Mg-alloy surface so as to contain the above-mentioned predetermined amount of calcium, manganese, and phosphorous, whereby a chemical conversion coating film having predetermined bare corrosion resistance, rust prevention, coating adhesion, and coating corrosion resistance can be obtained. In particular, calcium and phosphorous are considered to contribute to the enhancement of bare corrosion resistance of a chemical conversion coating film, and manganese is considered to contribute to the enhancement of coating adhesion.

Furthermore, the Mg or Mg-alloy surface is polished with the abrasive, whereby a draw, a void, a flow, and the like on the surface of a Mg or Mg-alloy housing product are eliminated, and it is not necessary to perform correction by puttying before coating, whereby a cost can be reduced. It is preferable that, due to the polishing function of the abrasive contained in the wet blast solution, the surface roughness of the Mg or Mg-alloy housing is set to be in a range of 0.5 μm to 100 μm in terms of a 10-point average surface roughness.

After the above-mentioned treatment, it is preferable that the surface is washed with water, followed by drying. Furthermore, if an aqueous liquid containing calcium ions, manganese ions, and phosphoric acid ions similar to those of the chemical conversion treatment agent used in wet blast, and an oxidation accelerator is shower-injected onto the surface after wet blast and washing with water, the thickness of the coating film can be increased to enhance corrosion resistance.

Thereafter, a coating treatment can be performed. In the coating treatment, it is possible that a coating agent such as epoxy resin is applied by spray coating, electrodeposition, etc., and a coating agent such as melamine resin is applied thereto. Alternatively a powdery coating agent of various kinds of resins can also be applied by electrostatic coating.

EXAMPLES

Hereinafter, the present invention will be described more specifically by way of an example and a comparative example.

Example 1

As a target member to be treated, a notebook computer housing (length: 329 mm, width: 274 mm, height: 2 mm) produced by subjecting Mg or a Mg-alloy of ASTM AZ91D to Thixo molding was used. Thixo molding was performed as follows: Mg or a Mg alloy in a semi-molten or molten state was extruded from an extruder at about 600° C. to a mold at about 250° C. to obtain a molded product. FIG. 1 shows a surface treatment process.

As a chemical conversion treatment agent, a mixture containing 80% by mass of “Grander-Finer MC1000” (produced by Million Chemical Co., Ltd.: 15 to 25% by mass of phosphoric acid, 10 to 15% by mass of manganese compound, and 10 to 20% by mass of calcium compound) and 20% by mass of alumina (Macorundum A#320, produced by Macoho Co., Ltd.) with an average particle size of 40 μm as an abrasive was used. Wet blast was performed using WFB-2-2C produced by Macoho Co., Ltd. under the following condition: a compression air pressure was 0.2 MPa, a pump pressure was 0.12 MPa, a distance between a housing and a nozzle was 30 mm, and the housing was fed forward at 40 mm/s. The housing was washed with water so as to remove the chemical conversion treatment agent and the abrasive, and further washed with deionized water, followed by drying.

Comparative Example 1

FIG. 2 shows a process of performing a chemical conversion treatment after etching as a comparative example. Surface adjustments 1 and 2 in FIG. 2 represent an etching treatment.

As a degreasing agent, “GFMG15SX” (produced by Million Chemical Co., Ltd.) was used, and the temperature thereof was kept at 70° C. Then, the above-mentioned Mg or Mg-alloy housing was soaked in the degreasing agent for 5 minutes to degrease the housing, followed by washing with water. As an etchant, “Grander-Finer MG104S” (produced by Million Chemical Co., Ltd.: 30 to 40% by mass of phosphoric acid, less than 0.1% by mass of surfactant, remaining water) was kept at a concentration of 5% and a temperature of 60° C. Then, the housing was soaked in the etchant for 60 seconds to perform etching, followed by washing with water. Thereafter, the member subjected to the etching treatment was soaked in the above-mentioned “GFMG15SX” (produced by Million Chemical Co., Ltd.) kept at a temperature of 60° C. for 7 minutes, followed by washing with water. As the chemical conversion treatment agent, “Grander-Finer MC 1000” (produced by Million Chemical Co., Ltd.: 15 to 25% by mass of phosphoric acid, 10 to 15% by mass of manganese compound, 10 to 20% by mass of calcium compound) was used, and kept at 35° C. Then, the Mg or Mg-alloy housing treated as described above was soaked in the chemical conversion treatment agent for 40 seconds, washed with water, and further washed with deionized water, followed by drying.

Example 1 and Comparative Example 1 were evaluated for an outer appearance. Furthermore, the adhesion amount of elements on the Mg or Mg-alloy housing surface was measured by quantitation with a fluorescent X-ray Five portions on the surface of the housing were assumed, and the variation thereof was checked. The electric resistance was measured by “Loresta MP” (four-terminal two-probe system) produced by Dia Instruments Co., Ltd. The results are shown in Table 1 and FIGS. 3 and 4. FIG. 3 is a photograph showing the outer appearance of Comparative Example 1, and FIG. 4 is a photograph showing the outer appearance of Example 1. The outer appearance of Example 1 had a uniform surface, whereas irregularity was observed on the outer appearance of Comparative Example 1.

TABLE 1 Electric Adhesion amount of Experiment Outer resistance elements (mg/m²) No. appearance Smut (Ω) Ca P Example 1 FIG. 3 None 0.4 24.4 ± 4  59.2 ± 7 Comparative FIG. 4 None 0.7 70.4 ± 35 112.6 ± 27 Example 1 Note 1: Smut refers to an outer appearance defect due to the adhesion of powder.

Furthermore, two-coat coating (baking at 150° C.) of an epoxy type was kept on a Mg or Mg-alloy housing in a high-temperature and high-humidity environment (60° C., relative humidity 95% RH) for 100 hours. Thereafter, a cross-cut test was performed (a checkerboard pattern was drawn on the surface of a sample to divide the surface into 100 portions, and the number of remaining grids after tape-up (peeling a tape) was counted to be determined based on the following standard: success when the number of remaining grids is 100, and failure when the number of remaining grids is less than 100.

FIGS. 5A to 5C are outer appearance photographs showing the results in Example 1, and FIGS. 6A to 6C are outer appearance photographs showing the results in Comparative Example 1. In FIGS. 5A-5C to 6A-6C, “OH” represents “before test”, and “2 mm” and “1 mm” respectively represent the size of each grid.

Furthermore, a salt spray test (SST) in accordance with JIS Z 2371 was performed for 8 hours and 24 hours, and corrosion resistance was evaluated. FIGS. 7A to 7C are outer appearance photographs showing the results in Example 1, and FIGS. 8A to 8C are outer appearance photographs showing the results in Comparative Example 1. In FIGS. 7A-7C to FIGS. 8A-8C, “OH” represents “before test”, and “8H” and “24H” respectively represent a test treatment time.

It was confirmed from the above results that the housing in Example 1 had a uniform color tone and was smooth, compared with Comparative Example 1. Furthermore, in Example 1, a draw, a void, a flow, and the like on the surface of the molded produce were eliminated. This enhanced the yield of coating performed after a treatment, and makes it unnecessary to perform correction by puttying before coating, so that a cost was reduced.

In Example 1, although the adhesion amount of a coating film was smaller than that in Comparative Example 1, the adhesion amount was less varied compared with that in Comparative Example 1. The other coating characteristics such as a resistance, a coating property and corrosion resistance exhibited similar values between Example 1 and Comparative Example 1.

Furthermore, in Example 1, since the number of processes is small, a coating treatment can be performed easily whereby a cost can be reduced.

-   -   The invention may be embodied in other forms without departing         from the spirit or essential characteristics thereof. The         embodiments disclosed in this application are to be considered         in all respects as illustrative and not limiting. The scope of         the invention is indicated by the appended claims rather than by         the foregoing description, and all changes which come within the         meaning and range of equivalency of the claims are intended to         be embraced therein. 

1. A method for producing a Mg or Mg-alloy housing, comprising: subjecting a surface of Mg or a Mg alloy to a wet blast treatment, using a mixture containing a chemical conversion treatment agent and an abrasive, thereby simultaneously forming unevenness due to the blast and a chemical conversion film on the surface of the Mg or Mg-alloy housing.
 2. The method for producing a Mg or Mg-alloy housing according to claim 1, wherein a surface roughness of the housing is in a range of 0.5 μm to 100 μm in terms of a 10-point average surface roughness.
 3. The method for producing a Mg or Mg-alloy housing according to claim 1, wherein the wet blast treatment includes injecting a mixture containing a liquid and an abrasive under a pressure in a range of 0.1 to 0.3 MPa.
 4. The method for producing a Mg or Mg-alloy housing according to claim 1, wherein the abrasive is at least one particulate material selected from the group consisting of alumina, zirconia, glass, and resin.
 5. The method for producing a Mg or Mg-alloy housing according to claim 4, wherein the resin is selected from the group consisting of melamine resin, urea resin, polyester resin, phenol resin, epoxy resin, and urethane resin.
 6. The method for producing a Mg or Mg-alloy housing according to claim 1, wherein an average particle diameter of the abrasive is in a range of 10 to 300 μm.
 7. The method for producing a Mg or Mg-alloy housing according to claim 1, wherein calcium used in a solution for the wet blast treatment is at least one selected from the group consisting of calcium nitrate, calcium nitrite, calcium thiosulfate, and tetracalcium phosphate.
 8. The method for producing a Mg or Mg-alloy housing according to claim 1, wherein manganese used in a solution for the wet blast treatment is at least one selected from the group consisting of manganese carbonate, manganese nitrate, manganese hydrogen phosphate, manganese biphosphate, and manganese fluoroborate.
 9. The method for producing a Mg or Mg-alloy housing according to claim 1, wherein phosphoric acid used in a solution for the wet blast treatment is at least one selected from the group consisting of orthophosphoric acid, condensed phosphoric acid, phosphorous acid, and hypophosphorous acid.
 10. The method for producing a Mg or Mg-alloy housing according to claim 1, wherein at least one selected from sodium chlorate and sodium hypochlorite is further added to a solution for the wet blast treatment as an oxidation accelerator.
 11. The method for producing a Mg or Mg-alloy housing according to claim 10, wherein an added amount of the oxidation accelerator is in a range of 0.02 to 2 g/L.
 12. The method for producing a Mg or Mg-alloy housing according to claim 1, wherein a proportion of the chemical conversion treatment agent of a solution for the wet blast treatment is 70 to 90% by mass, and a proportion of the abrasive of the solution for the wet blast treatment is 10 to 30% by mass. 